The embryotoxic, cytogenetic and carcinogenic effect of new as well as clinically useful antitumor agents are under investigation. The embryotoxicity of two adenosine deaminase inhibitors, 2'-deoxycoformycin and EHNA, alone and in combination with xylosyladenine and ara-A has been investigated in mice, and studies on the embryotoxic and cytogenetic effects of two inhibitors of de novo pyrimidine biosynthesis (PALA and 6-azauridine) have been completed. The embryotoxic effects of anthracenedione, dihydroxyanthracenedione, and spirogermanium are also being evaluated in mice. The carcinogenic activity of various clinically effective antitumor and/or immunosuppressive agents, including procarbazine, N-methylnitrosourea, mephalan, azathioprine, adriamycin, and cyclophosphamide, is under evaluation in long-term studies in monkeys. A survey of second malignancies developing in treated cancer patients, initiated several years ago, is continuously being updated using data from published reports and from several centers and hospitals.